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Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory Molecular Orbital Theory

Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

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Page 1: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

Chemical Bonding

Lewis Theory-VSEPR Valence Bond Theory

Molecular Orbital Theory

Page 2: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

VB theory predicts properties better than Lewis theory

bonding schemes, bond strengths, lengths, rigidity

There are still properties it doesn’t predict perfectly

magnetic behavior of certain molecules

strength of bonds

VB theory presumes the electrons are localized in orbitals

doesn’t account for delocalization

Problems with Valence Bond Theory

Page 3: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

Molecular Orbital Theory

In MO theory, we apply Schrödinger’s wave equation to the molecule to calculate a set of molecular orbitals.

The equation solution is estimated .

We start with good guesses as to what the orbitals should look like, then test the estimate until the energy is minimized

The electrons belong to the whole molecule

orbitals are delocalized

Page 4: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

LCAO

The simple guess starts with atomic orbitals of the atoms adding together to make molecular orbitals,

the Linear Combination of Atomic Orbitals.

The waves can combine either constructively or destructively.

Page 5: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

Molecular Orbitals

When wave functions combine constructively, the resulting molecular orbital has less energy than the original atomic orbitals

it is called a Bonding Molecular Orbital σ, π most of the electron density between the nuclei

Amplitudes of wave functions added

Page 6: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

Molecular Orbitals

When wave functions combine destructively, the resulting molecular orbital has more energy than the original atomic orbitals

it is called an Antibonding Molecular Orbital σ*, π* most of the electron density outside the nuclei nodes between nuclei

Amplitudes of wave functions subtracted.

Page 7: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

Interaction of 1s Orbitals

Increasing energy

σ1s*antibonding

molecular orbital

σ1sbonding

molecular orbital

1s 1s

destructive interference

-

constructive interference

+

Page 8: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

Molecular Orbital Theory

Electrons in bonding MOs are stabilizing.

lower energy than the atomic orbitals

Electrons in antibonding MOs are destabilizing.

higher in energy than atomic orbitals electron density located outside the internuclear axis electrons in antibonding orbitals cancel stability

gained by electrons in bonding orbitals

Page 9: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

Molecular Orbitals and Properties

Bond Order = difference between number of electrons in bonding and antibonding orbitals

only need to consider valence electrons may be a fraction higher bond order = stronger and shorter bonds

If bond order = 0, then bond is unstable compared to individual atoms and no bond will form.

A substance will be paramagnetic if there are unpaired electrons in molecular orbitals.

Page 10: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

A Molecular Orbital Diagram - H2

1s atomic orbital

σ

σ*

bonding MO

antibonding MO

1s atomic orbital

H· ·H

Page 11: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

A Molecular Orbital Diagram - H2

1s atomic orbital

σ

σ*

HOMO

LUMO

1s atomic orbital

H· ·H

highest occupied molecular orbital

lowest unoccupied molecular orbital

Page 12: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

A Molecular Orbital Diagram - H2

1s atomic orbital

σ

σ*

1s atomic orbital

H· ·H

Because more electrons are in bonding orbitals than are in antibonding orbitals,

there is a net bonding interaction.

= 1BO = ½( be – abe)

Page 13: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

A Molecular Orbital Diagram - He2

1s atomic orbital

σ

σ*

1s atomic orbital

He: He:

Because as many electrons are in bonding orbitals as in antibonding orbitals,

NO NET BONDING INTERACTION.

= 0

X

BO = ½( be – abe)

Page 14: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

A Molecular Orbital Diagram - Li2

2s atomic orbital

σ

σ*

2s atomic orbital

Li· ·Li

1s atomic orbital σ

σ*

1s atomic orbital

Page 15: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

A Molecular Orbital Diagram - Li2

2s atomic orbital

σ

σ*

2s atomic orbital

Li· ·Li

Because more electrons are in bonding orbitals than are in antibonding orbitals,

there is a net bonding interaction.

= 1

Page 16: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

2s atomic orbital

σ

σ*

2s atomic orbital

Be: :Be

MO Diagram - Be2

Because as many electrons are in bonding orbitals as in antibonding orbitals,

NO NET BONDING INTERACTION.

= 0

Page 17: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

Interaction of p Atomic Orbitals

Each time we combine two atomic orbitals, we obtain two molecular orbitals: one bonding and one antibonding.

In the case of p atomic orbitals, overlap occurs either "head-to-head" to form σ and σ* molecular orbitals or “sideways" to form π and π*

molecular orbitals.

Page 18: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

2pz

𝝈2p*

𝝈2p

2pz

+

Atomic orbitals

Molecular orbitals

“bonding”

“antibonding”

Page 19: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

+

2px 2px

+

2py 2py

π2p*

+

π2p

π2p*

+

π2p

Atomic orbitals Molecular orbitals“antibonding” “bonding”

Page 20: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

Molecular Orbitals

B2, C2, N2, O2, F2, Ne2,

Page 21: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

2s atomic orbital

σ2s

σ2s*

2s atomic orbital

MO Diagram - B2

B B

σ2p*

σ2p

π2p

π2p*

2p atomic orbital2p atomic orbital

BO = ½(6 be – 4 abe) BO = 1

Page 22: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

2s atomic orbital

σ2s

σ2s*

2s atomic orbital

MO Diagram - C2 σ2p*

σ2p

π2p

π2p*

2p atomic orbital2p atomic orbital

BO = ½(8 be – 4 abe) BO = 2

C C

Page 23: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

2s atomic orbital

σ2s

σ2s*

2s atomic orbital

MO Diagram - N2 σ2p*

σ2p

π2p

π2p*

2p atomic orbital2p atomic orbital

BO = ½(10 be – 4 abe) BO = 3

N N

Dinitrogren ( N2 ) is Diamagnetic!!

Page 24: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

2s atomic orbital

σ2s

σ2s*

2s atomic orbital

MO Diagram - C22-

(carbide ion) σ2p*

σ2p

π2p

π2p*

2p atomic orbital2p atomic orbital

BO = ½(10 be – 4 abe) BO = 3

C C

Page 25: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

σ2p*

σ2p

π2p

π2p*

2p atomic orbital2p atomic orbital

σ2p*

σ2p

π2p

π2p*

2p atomic orbital2p atomic orbital

Molecular Orbitals B2, C2, N2

Molecular Orbitals O2, F2, Ne2

Page 26: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

2s atomic orbital

σ2s

σ2s*

2s atomic orbital

MO Diagram - O2 σ2p*

σ2p

π2p

π2p*

2p atomic orbital2p atomic orbital

BO = ½(10 be – 6 abe) BO = 2

O O

Page 27: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

Dioxygen ( O2 ) is Paramagnetic !!

Page 28: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

2s atomic orbital

σ2s

σ2s*

2s atomic orbital

MO Diagram - F2 σ2p*

σ2p

π2p

π2p*

2p atomic orbital2p atomic orbital

BO = ½(10 be – 8 abe) BO = 1

F F

Page 29: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

2s atomic orbital

σ2s

σ2s*

2s atomic orbital

MO Diagram - Ne2 σ2p*

σ2p

π2p

π2p*

2p atomic orbital2p atomic orbital

BO = ½(10 be – 10 abe) BO = 0

Ne Ne

X

Page 30: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

Using MO Theory to Explain Bond Properties

As the following data show, removing an electron from N2 forms an ion with a weaker, longer bond than in the parent molecules, whereas the ion formed from O2 has a stronger, shorter bond:

These facts can be explained by examining diagrams that show the sequence and occupancy of MOs.

Page 31: Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory ...profkatz.com/.../2017/03/CH1710-Lecture-23-MO-Theory-web-copy.pdf · Chemical Bonding Lewis Theory-VSEPR Valence Bond Theory

σ2s

σ2s*

σ2p*

σ2p

π2p

π2p*

σ2s

σ2s*

σ2s

σ2s*

σ2p*

σ2p

π2p

π2p*

N2 N2+ O2 O2+

B.O.=3 B.O.=2.5 B.O.=2 B.O.=2.5

Electron is lost from bonding molecular orbital.

Electron is lost from anti- bonding molecular orbital.

σ2s

σ2s*